Catalytic dehydrogenation is an industrial process that produces C3-C4 range olefins such as propylene, butenes (1-butene, 2-butene, or isobutene), and 1,3-butadiene from its corresponding light alkane feed. The light alkane feed comprises propane, butane, or a combination thereof. A stable supply of light alkane feedstock at competitive prices makes dehydrogenation a preferred process for C3-C4 olefin production compared to other production processes such as steam cracking of naphtha or fluid catalytic cracking (FCC) of heavy-portion of crude oil.
Light alkane dehydrogenation is an equilibrium-limited and highly endothermic reaction. Equilibrium conversion of light alkane feed increases with temperature and decreases with pressure. Cracking and coking reactions, promoted by elevated temperature as side reactions, reduce the selectivity toward the desired olefins.
Light alkane dehydrogenation is a strongly endothermic reaction and, therefore, the process for producing its corresponding olefin from the light alkane feed requires supplying a large quantity of reaction heat to achieve industrially attractive production rates. Approximately 2,952 kJ of thermal energy is required for the reaction heat per kg of propylene produced from propane. Considering this strong endothermic requirement, there is a need for a reliable and efficient method and apparatus for providing the reaction heat required for producing olefins from the light alkanes at industrially attractive production rates.
The present invention found that another important factor in olefins production is uniform catalyst bed temperature in a specific temperature range. Findings of the present invention suggest that light alkane conversion for olefin production is highly sensitive to reaction temperature in terms of light alkane conversion rates and catalyst deactivation. If the catalyst bed temperature is below 500° C., light alkane conversion rate is too low to meet commercially attractive production rates. On the other hand, unacceptably fast catalyst deactivation is driven at catalyst bed temperatures higher than 700° C., and this leads to olefin production cycle times between catalyst regenerations that are too short for commercial operation. Achieving a uniform catalyst bed temperature in a desired temperature range, preferably between 500° C. and 700° C., more preferably between 520° C. and 680° C., and most preferably between 540° C. and 660° C., in industrial scale reactors is critical for commercial viability of olefin production from light alkanes.
Methods for supplying reaction heat have been developed by chemical industry for reactions of endothermic nature. However, adoption of these methods for dehydrogenation of light alkanes yields undesirable operational issues and non-uniform temperature distributions in the catalyst bed. For instance, preheating the light alkane feedstock to provide sufficient sensible heat for the endothermic reaction is not feasible because the reaction heat required for industrially attractive rates is substantially larger than the quantity of sensible heat achievable through light alkane feedstock preheating. Excessive preheating of the feedstock in order to increase sensible heat and provide the reaction heat required often leads to technical issues, including thermal breakdown of feedstock, accelerated catalyst deactivation, and shortened lifetime of preheating tubes. Heating an inter-stage stream for the next stage reactor in serially connected multi-stage reactors configuration is not desirable either because heating of the inter-stage stream leads to thermal breakdown of the desired product at elevated temperatures and its resultant building-up of coke inside the tube.
Intensive heating-up of external surfaces of fixed bed reactors would not be applicable. Catalyst beds with a fixed position in a stationary state inside an externally heated reactor impede heat supply itself and create non-uniform temperature distributions within the catalyst bed. This leads to accelerated catalyst coking and catalyst sintering problems near the reactor wall and not enough thermal energy to drive the endothermic reaction in the center of the catalyst bed.
Catalyst heating by burning coke while regenerating catalyst (and burning extra fuel when needed) and recycling the heated catalyst for reaction heat supply has been explored. Even though circulation of heated catalyst particles from the catalyst regenerator for reaction heat supply has been commercially employed in fluid catalytic cracking (FCC) for heavy portions of crude oil, the same approach or its modified approach (e.g., back-mixing of the regenerated catalyst particles) would not work properly with light alkanes as feedstock. Light alkane conversion for the production of olefins requires substantially larger amounts of reaction heat than cracking of heavy portions of crude oil when compared on a per unit feedstock mass basis. The present invention also found that light alkane feed produces coke at substantially lower yields than FCC methods for heavy portions of crude oil. The much stronger endothermic requirement of light alkane dehydrogenation combined with substantially lower coke yield in the reaction makes it impractical to use coke as source of reaction heat supply.
Catalyst deactivation driven by coke formation is another technical hurdle in olefin production from light alkanes. Formation of coke over or within the catalyst structure progresses over the course of olefin production, leading to a gradual drop in olefin production rates. Regeneration of deactivated catalysts makes it difficult or impossible to produce olefins from a reactor in a continuous manner and to operate downstream separation systems or units without interruption.
Taken together, there is a need for a new process and apparatus for producing olefins from a light alkane feedstock by developing a reliable and efficient reaction heat supply to the reactor with a uniform catalyst bed temperature in a desired temperature range and by making the entire process continuous.